As a distance measuring device, a light wave distance measuring device is known. In the light wave distance measuring device, a laser beam is projected to an object to be measured and a distance to the object to be measured is determined by using a reflection light from the object to be measured.
In the light wave distance measuring device known in the past, intensity of the laser beam is modulated at a fixed frequency, and the laser beam is projected as a distance measuring light. A reflected distance measuring light reflected by the object to be measured is received. A phase of intensity modulation of the reflected distance measuring light thus received is compared with a phase of intensity modulation of an internal reference light obtained on a reference optical path formed within the distance measuring device, and a distance to the object to be measured is determined from the phase difference.
The distance measurement in the distance measuring device as described above is based on the phase difference, which is changed depending on the measured distance. The phase difference Δφ is expressed by the following equation 1:Δφ=4πfD/C  (Equation 1)where Δφ represents phase difference between the internal reference light and the reflected distance measuring light, D represents a measured distance, f represents modulation frequency, and C is light velocity.
The measured distance D can be determined by measuring the phase difference Δφ. Because the length of optical path for reference is already known, an accurate distance can be determined through correction of the measured distance by internal reference light optical path.
Also, in the distance measurement, drift of detection circuit or the like within the distance measuring device exerts influence and causes measurement error. By comparing the phase of the internal reference light with the phase of the reflected distance measuring light, the influence of the drift of the detection circuit or the like can be offset, and accurate distance can be calculated.
Next, description will be given on general features of a conventional type distance measuring device referring to FIG. 7.
A light emitting element 1 such as a laser diode emits a laser beam, and intensity of the laser beam is modulated at a predetermined frequency by means of a light emission driving circuit 12. The laser beam is split to a distance measuring light 3 and an internal reference light 4 by a half-mirror 2. The distance measuring light 3 transmits the half-mirror 2, and then the distance measuring light 3 is projected to an object 6 to be measured, e.g. a reflection mirror such as a corner cube, through an objective lens 5. A reflected distance measuring light 3′ reflected by the object 6 to be measured is received by a photodetection element 7 such as a photo-diode through the objective lens 5 and a half-mirror 8.
The internal reference light 4 is reflected by the half-mirror 2 and the internal reference light 4 is reflected further by the half-mirror 8 on the optical path of the reflected distance measuring light 3′ and the internal reference light 4 is received by the photodetection element 7. A photodetection signal of the photodetection element 7 is inputted to a photodetection circuit 13, and the photodetection circuit 13 processes the signal inputted from the photodetection element 7 for calculating a measured distance.
An optical path switcher 9 is provided, which stretches over the optical path of the distance measuring light 3 and the optical path of the internal reference light 4. And a light amount adjuster 11 is provided on the optical path of the reflected distance measuring light 3′. The optical path switcher 9 selectively cuts off one of the optical paths of the distance measuring light 3 and of the internal reference light 4, and allows the other of the optical paths of these lights to pass. At the photodetection element 7, the reflected distance measuring light 3′ and the internal reference light 4 are received alternately.
As described above, the distance measuring light 3 with intensity modulation is used, and distance is calculated by obtaining phase difference between the internal reference light 4 and the reflected distance measuring light 3′ obtained from the distance measuring light 3. Thereby the difference of the photodetection light amount between the reflected distance measuring light 3′ and the internal reference light 4 exerts influence on the accuracy of the distance measurement. Therefore the light amount adjuster 11 is provided. The light amount adjuster 11 comprises an amplitude filter with density continuously changing. By rotating the amplitude filter, the photodetection light amount of the reflected distance measuring light 3′ can be adjusted to a constant level. By the light amount adjuster 11, it is so designed that the photodetection light amount of the internal reference light 4 received by the photodetection element 7 is equal to the photodetection light amount of the reflected distance measuring light 3′ received by the photodetection element 7 even when the reflected light amount is changed depending on the distance to the object 6 to be measured.
Optical path switching by the optical path switcher 9 and the light amount adjustment by the light amount adjuster 11 are controlled by the driving circuit 14.
The control arithmetic unit 15 controls the light emission driving circuit 12 so that the laser beam emitted from the light emitting element 1 has an intensity modulation at a predetermined frequency, and controls the timing by optical path switching by the optical path switcher 9 which is driven by the driving circuit 14. Further, from the photodetection signal of the photodetection element 7, the control arithmetic unit 15 sends out a control signal to the driving circuit 14 to equalize light amount of the reflected distance measuring light 3′ to light amount of the internal reference light 4.
The photodetection circuit 13 performs signal processing such as amplifying and A/D conversion on the signal from the photodetection element 7 and also performs processing such as obtaining phase difference between modulated frequency of the internal reference light 4 and modulated frequency of the reflected distance measuring light 3′, and the result is sent out to the control arithmetic unit 15. Based on the phase difference sent out from the photodetection circuit 13, the control arithmetic unit 15 calculates a distance to the object 6 to be measured by using the above equation 1.
In the distance measuring device as described above, the internal reference light 4 and the reflected distance measuring light 3′ are mechanically switched over by the optical path switcher 9.
Because optical path switching and light amount adjustment are both performed mechanically, it is difficult to carry out optical path switching and light amount adjustment at high speed, therefore it is difficult to perform distance measurement at high speed. For this reason, although there is no problem when distance is measured on an object to be measured such as a building, the measurement may be difficult to perform when high speed distance measurement is required, e.g. in case distance should be measured continuously on a plurality of moving objects, such as bulldozers, by using a single measuring device. Also, in case that 3-dimensional measurement is performed on an object such as building by a total station etc., surveying must be carried out on a multiple points by automatic surveying, and measurement must be performed at high speed. In case that surveying is carried out on a moving object etc., optical path switching speed and light amount adjusting speed cannot follow the moving speed of the moving object, and there are problems that distance cannot be measured in such cases.
Distance measuring device for measuring distance in multiple directions and at multiple points by rotating the distance measuring light are disclosed in the Japanese Patent Publication No. 2694647 and the Japanese Patent Publication Laid-Open 4-313013.
To solve the above problems, it is an object of the present invention to provide a distance measuring device, by which it is possible to perform optical path switching and light amount adjustment at high speed in the distance measuring device and to accomplish distance measurement at high speed.